E-mail an hourly update
Instead of e-mailing every single occurrence of a liked post or comment, you could just send an e-mail every hour if there is new activity to share. If there is no activity to share, don't send an e-mail. It would take some load off you system from all the e-mails and not clutter our e-mail boxes like Facebook oh so loves to do.
Thanks for the suggestion
For reference, we currently check for new items to email every 10 minutes. We group together earned badges into a single email to reduce the number of emails.
We will look into doing the same for the other types to reduce those as well.